This invention relates to a method of manufacturing an oriented silicon steel sheet having improved magnetic characteristics and, more particularly, to an improved method of preventing reduction of magnetic flux density notwithstanding reduction of thickness of the silicon steel sheet.
High magnetic flux density and a small core loss are magnetic characteristics required in grain-oriented silicon steel sheets. Recent progress in manufacture techniques has made it possible to make, for example, a silicon steel sheet having a magnetic flux density B.sub.8 (the value at a magnetizing force of 800 A/m) of 1.92 T for a sheet having a thickness of 0.23 mm. It is also possible to manufacture, on an industrial scale, an improved silicon steel sheet product having a core loss characteristic W.sub.17/50 (value under a fully magnetized condition: 1.7 T at 50 Hz}of 0.90 w/kg.
Silicon steel sheets having such improved magnetic characteristics have crystalline structures in which the &lt;001&gt; directions parallel to the axis of easy magnetization are uniformly aligned in the direction of rolling of the steel sheet. Such a texture is formed during finishing annealing by a phenomenon called secondary recrystallization in which crystal grains having a (110) [001] direction called the Goss direction are grown with priority into giant grains. Fundamental requirements for effectively growing secondary recrystallized grains include the existence of an inhibitor for limiting the growth of crystal grains having undesirable directions other than the (110) [001]direction in the secondary recrystallization process and the formation of a primary recrystallized crystalline structure suitable for effectively developing secondary recrystallized grains in the (110) [001] direction.
A fine precipitate of MnS, MnSe, AlN or the like is ordinarily utilized as the inhibitor. The effect of the inhibitor has been enhanced by adding a grain boundary segregation type component such as Sb or Sn to the inhibitor. Conventionally, methods in which MnS or MnSe is used as a main inhibitor are advantageous in reducing the core loss of certain sheets because they assist in reducing the sizes of the secondary recrystallized grains. However, methods based on laser irradiation or plasma jetting have recently been provided to artificially form pseudo grain boundaries so that the magnetic domains are fractionated and the core loss is reduced. For this reason the advantage of reducing the sizes of the secondary recrystallized grains has been lost. Further, the concept of increasing the magnetic flux density of the steel sheet has become advantageous.
A method of manufacturing an oriented silicon steel sheet having a large magnetic flux density is disclosed in Japanese patent Publication 46-23820. According to this method, the desired steel sheet can be manufactured by (a) introducing Al into the steel as an inhibitor component, (b) quenching to obtain cooling before final cold rolling to precipitate AlN, and (c) increasing the rolling reduction of the final cold rolling from a lower reduction to a higher reduction, like from 65 to 95%.
The method of the Japanese Publication, however, entails a problem in that the magnetic flux is abruptly reduced along with the reduction of thickness of the product sheet. It is very difficult or impossible to manufacture by the method of the Japanese Publication the type of silicon steel sheet presently in demand, e.g., a thin product having a thickness of 0.25 mm or less and having a B.sub.8 value of 1.94 T or higher.
In Japanese patent Publication 46-23820, immersing a steel sheet in hot water at 100.degree. C. after annealing to quench the sheet is disclosed, but there is no consideration or mention of any phase of any carbides after quenching. Ordinarily, in the case of slow cooling from 600.degree. C. or lower, carbides are precipitated from grain boundaries at a higher temperature and are precipitated in crystal grains at a lower temperature. Carbides precipitated are finer and have a higher density if precipitation is started at a reduced temperature. Accordingly, with respect to the first embodiment of Japanese patent Publication 46-23820 in which the time for cooling from 1,000 to 750.degree. C. is about 10 seconds and the time for cooling from 750 to 100.degree. C. is about 25 seconds, it is not unreasonable to conclude that very fine carbides having particle sizes of several tens of angstroms are precipitated or that the extent of carbide precipitation is limited and that the carbon is simply supersaturated in the steel.
Japanese Patent Publication 56-3892 discloses a technique for controlling carbides in other steels during cooling after annealing. In this method, with respect to two-stage cold rolling, the steel is cooled at a cooling speed of 150.degree. C./min or higher from 600 to 300.degree. C. during cooling after annealing followed by final cold rolling so that the amount of solid solution carbon after cooling is increased. This method is intended to improve the magnetic characteristics of the steel by increasing the amount of solid solution carbon in the steel and by optimizing the aging effect between cold rolling paths. Such an effect of solid solution carbon is well known in the case of ordinary cold-rolled steel sheets. If the amount of solid solution C or solid solution N before cold rolling is increased, the (110) intensity in the recrystallized structure formed by recrystallization annealing after cold rolling is increased. In the case of oriented silicon steel sheets, the (110) grains become nuclei for secondary recrystallization, so that the number of secondary recrystallized grains is increased, the secondary-recrystallized grains are finer, and improved magnetic characteristics can be achieved. This method, however, does not enable the magnetic flux density of a thin oriented silicon steel sheet to be increased.
As a technique for controlling the form of C in steel to increase the (110) intensity of the steel, a method of precipitating many fine carbide grains during cooling after intermediate annealing is disclosed in Japanese Patent Laid-Open Publication 58-157917. In this method, quenching of the steel to 300.degree. C. is effected after intermediate annealing and slow cooling is applied for 8 to 30 seconds through a temperature range of 300 to 150.degree. C., thereby precipitating fine carbides. The (110) intensity of the steel after recrystallization is thereby increased so that the magnetic characteristics of the steel are improved. However, the magnetic characteristics achieved by these methods are at most 1.94 T with respect to B and 1.92 T with respect to B.sub.8 when the sheet thickness is 0.3 mm, which value is not high enough to be satisfactory.
Japanese Patent Laid-Open Publication 61-149432 discloses a technique based on setting the cooling speed of steel to 10.degree. C./s or higher at the time of cooling after intermediate annealing, creating a work strain of 1 to 30 % during cooling from 1,000 to 400.degree. C., and performing finishing rolling at a temperature in the range of 100.degree. C. to 400.degree. C. According to this method, a work strain of 1 to 30 % is created at a temperature in the range of 1,000 to 400.degree. C. in which the C diffusion speed is very high to provide high-density dislocations, so that C is finely precipitated at the dislocations and the (110) intensity is increased. To finely precipitate C in dislocations at a high density, the working is performed by rolling, and a high cooling speed of 10.degree. C./s or higher is set for the precipitation step. The core loss can be reduced to a certain extent by this method but the magnetic flux density achieved by this method is only 1.91 T with respect to B.sub.10 (1.89 T with respect to B.sub.8), which is low.